Method and apparatus for air separation by cryogenic distillation

ABSTRACT

The invention relates to a method for air separation by cryogenic distillation in a column system that includes at least a first column operating at a first pressure, and a second column operating at a second pressure that is lower than the first pressure, the head of the first column being thermally connected to the tank of the second column via a vaporizer-condenser, a first portion of a nitrogen-enriched gas is drawn from the head of the first column, compressed in a compressor having an inlet temperature no higher than −150° C. and condensed in the vaporizer-condenser, an oxygen-rich fluid is drawn from the lower portion of the second column and heated in the exchange line, a nitrogen-rich gas is drawn from the upper portion of the second column and heated in the exchange line, and a second portion of the nitrogen-enriched gas is expanded in a turbine without being compressed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a §371 of International PCT Application PCT/FR2013/050396, filed Feb. 27, 2013, which claims the benefit of FR1254340, filed May 11, 2012, both of which are herein incorporated by reference in their entireties.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a process and to an apparatus for separating air by cryogenic distillation. The described invention proposes in particular a solution for optimizing the efficiency of an air separation unit producing high-purity oxygen (greater than 97% O₂). All the percentages relating to purities in this document are molar percentages.

It proposes, in order to maximize the efficiency of the cycle, to cold compress (that is to say compress it in a compressor having an inlet temperature of at most −150° C.) the nitrogen flow escaping from the top of a first column, referred to as the medium-pressure (MP) column and sent to the main vaporizer-condenser of the distillation system, thermally coupling the top of the medium-pressure (MP) column and the bottom of a second column, referred to as the low-pressure (LP) column. This cold compression makes it possible to reduce the discharge pressure of the main air compressor. The nitrogen may be compressed without having been reheated in the exchange line or else it may be partially reheated in the exchange line before being compressed in the main exchange line, then optionally cooled again before being sent to the vaporizer-condenser.

The envisaged application is the installation of one or more air separation units that produce pure oxygen.

BACKGROUND

It is known to use a cold compressor for heating nitrogen intended for a bottom vaporizer-condenser of a low-pressure air separation column. In the prior art, another portion of the nitrogen originating from the medium-pressure column is used to vaporize the oxygen-enriched liquid originating from the medium-pressure column or else to heat an intermediate vaporizer of the low-pressure column.

The processes for oxygen-consuming clients may require, in certain types of applications, a high purity (typically of the order of 99.5%) at low or high pressure.

In order to reduce the electricity consumption of the unit, it is possible to seek to reduce the discharge pressure of the main compressor as much as possible, which corresponds, in conventional layouts and excluding the pressure drops, to the pressure of the medium-pressure (MP) column.

In order to do this, a conventional double column layout (optionally with an argon column), producing oxygen at a purity >97% (i.e. containing only oxygen and argon, and no longer any nitrogen at all) is considered, in which the refrigeration needed for keeping the apparatus cold is provided by expansion of air or nitrogen in a cryogenic turbine. Usually, for such an oxygen purity, the heat exchange between the MP column and the low-pressure (LP) column is provided by a single vaporizer-condenser, the main vaporizer-condenser of the unit.

The installation of a cold compressor is then considered, that compresses all of the nitrogen flow withdrawn at the top of the MP column, which is required in order to operate the main vaporizer-condenser of the unit. The role of this cold compressor is therefore to decorrelate the pressure of the MP column from the pressure of nitrogen in the vaporizer-condenser which makes it possible to reduce the pressure of the MP column below the pressure required for carrying out the heat exchange between nitrogen and oxygen in the vaporizer-condenser. The pressure of the main air compressor is therefore reduced.

This optimization can only be achieved from the moment when the turbines produce sufficient refrigeration so that the use of the cryogenic compressor does not prevent the apparatus from being kept cold. In particular, the production of cryogenic liquids (liquid oxygen, nitrogen, argon) “consumes” the available refrigeration and therefore reduces the effect of the cryogenic compressor.

It should also be noted that this device may be applied to existing sites where the existing air (and where appropriate oxygen) compressors are reused and where only the cold box is replaced. This optimization makes it possible to greatly reduce the operating pressure of the air compressors, as far as is permitted by the performance curves of the machine.

A conventional double column does not make it possible to lower the pressure of the main compressor by decorrelating the pressure of the main vaporizer-condenser from the pressure of the main compressor.

In other layouts, the cold compressor handles nitrogen intended for the upper vaporizer-condenser, which vaporizes a very rich liquid (around 70% O₂) at an intermediate level in the LP column. The LP column bottom vaporization is carried out by a second vaporizer-condenser, operating with liquid air. The main production of oxygen is not withdrawn level with the vaporizer supplied by the cryogenic compressor. Moreover, this layout is only of use for the production of oxygen having a purity of less than 97%.

JP-A-54020986 describes a process according to the preamble of claim 1. Here, all the nitrogen expanded in the turbine was compressed in the cold compressor. However, the energy consumption of this process is 3% greater than that of the present invention and the prior art process also entails using larger compressors.

SUMMARY OF THE INVENTION

According to one subject of the invention, a process is provided for separating air by cryogenic distillation in a system of columns comprising at least a first column operating at a first pressure and a second column operating at a second pressure below the first pressure, the top of the first column being thermally coupled to the bottom of the second column by a vaporizer-condenser wherein:

i) purified air is cooled in an exchange line and is sent to the first column in order to be separated therein,

ii) a nitrogen-enriched gas is withdrawn from the top of the first column and divided into two portions, a first portion is compressed in a compressor having an inlet temperature of at most −150° C. and is condensed in the vaporizer-condenser,

iii) a second portion of the nitrogen-enriched gas is reheated in the exchange line and expanded in an expansion turbine,

iv) an oxygen-enriched liquid or a liquid derived from this liquid is sent from the bottom of the first column to the second column in order to be separated therein, without having been reheated against a nitrogen-enriched gas from the first column,

v) a nitrogen-enriched liquid is sent from the top of the first column to the top of the second column,

vi) an oxygen-rich fluid is withdrawn from the bottom of the second column and is reheated in the exchange line,

vii) a nitrogen-rich gas is withdrawn from the upper part of the second column and is reheated in the exchange line,

viii) the vaporizer-condenser being the only means of vaporizing liquid in the lower part of the second column,

characterized in that the second portion of the nitrogen-enriched gas is reheated in the exchange line and is expanded in the turbine, without having been compressed.

According to other optional subject matters:

-   -   the oxygen-rich fluid contains at least 97% oxygen, or even at         least 99% oxygen.     -   the oxygen-rich fluid from step v) is a liquid that is vaporized         and reheated in the exchange line.     -   the vaporized liquid is compressed after preheating in the         exchange line.     -   the nitrogen-enriched liquid sent from the first column to the         second column is less rich in nitrogen than the         nitrogen-enriched gas sent to the vaporizer-condenser.     -   all the gas condensed in the vaporizer-condenser is sent back as         reflux to the first column.     -   the oxygen-rich fluid from step v) is an oxygen-rich gas.     -   the oxygen-rich gas is compressed after reheating in the         exchange line to its production pressure.     -   an argon-enriched gas is withdrawn from the second column and         sent to a third column in order to produce a flow richer in         argon than the argon-enriched gas and a nitrogen-rich gas is         withdrawn from the first column as product.     -   the compressor is driven by a motor and/or by a turbine that         expands air or a nitrogen-enriched flow originating from the         system of columns.     -   during a first operating mode, the compressor is shut down and         at least one liquid is withdrawn from the system of columns as         final product and during a second operating mode, the compressor         operates and no liquid is withdrawn from the system of columns         as final product or at least three times less liquid is         withdrawn from the system of columns as during the first         operating mode.     -   the first operating mode is used only if the price of         electricity is below a threshold and the second operating mode         is used only if the price of electricity is above the threshold.

According to another subject of the invention, an apparatus is provided for separating air by cryogenic distillation that has a system of columns comprising at least a first column operating at a first pressure and a second column operating at a second pressure below the first pressure, the top of the first column being thermally coupled to the bottom of the second column by a vaporizer-condenser, an exchange line, means for sending purified air to be cooled into the exchange line and for sending the cooled air to the first column in order to be separated therein, means for withdrawing a nitrogen-enriched gas from the top of the first column, a compressor for compressing a first portion of the nitrogen-enriched gas having an inlet temperature of at most −150° C., means for sending a second portion of the nitrogen-enriched gas to the exchange line, a turbine for expanding the second portion of the nitrogen-enriched gas that is connected to the exchange line, means for sending the compressed gas to be condensed into the vaporizer-condenser, means for sending an oxygen-enriched liquid or a liquid derived from this liquid from the bottom of the first column to the second column in order to be separated therein, means for sending a nitrogen-enriched liquid from the top of the first column to the top of the second column, means for withdrawing an oxygen-rich fluid from the bottom of the second column, means for sending the oxygen-rich fluid to be reheated into the exchange line, means for withdrawing a nitrogen-rich gas from the upper part of the second column, means for sending the gas withdrawn to be reheated into the exchange line, any means for reheating the oxygen-enriched liquid to the second column against a nitrogen-enriched gas from the first column and the vaporizer-condenser being the only means of vaporizing liquid in the lower part of the second column.

According to other optional subject matters, the apparatus comprises:

-   -   means for withdrawing the nitrogen-enriched liquid sent from the         first column to the second column to a lower level of the first         column than the nitrogen-enriched gas sent to the         vaporizer-condenser.     -   means for sending all the gas condensed in the         vaporizer-condenser as reflux to the first column.     -   the oxygen-rich fluid is an oxygen-rich gas.     -   the apparatus comprises an oxygen compressor in order to         compress the oxygen-rich gas, after reheating in the exchange         line, to its production pressure.     -   the apparatus comprises a third column, means for sending an         argon-enriched gas from the second column to the third column,         means for withdrawing a flow richer in argon than the         argon-enriched gas from the third column and means for         withdrawing a nitrogen-rich gas from the first column as         product.     -   the compressor is driven by a motor and/or by a turbine that         expands air or a nitrogen-enriched flow originating from the         system of columns.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

FIG. 1 provides an embodiment of the present invention.

FIG. 2 provides an embodiment of the present invention.

DETAILED DESCRIPTION

The invention will be described in greater detail by referring to the figures which represent processes according to the invention.

In FIG. 1, the system of columns only comprises a double column with a first column 100 and a second column 102 operating at a lower pressure than that of the first column. The columns are integrated together solely by a bottom condenser 21 of the second column, heated by the overhead nitrogen 17 from the first column 100 after compression in a cold compressor 19.

A flow of air 1 at the pressure of the first column supplies the first column after cooling in a heat exchanger 9.

An oxygen-enriched liquid 35 is sent from the bottom of the first column to the second column after cooling in a subcooler 104. A nitrogen-enriched liquid 37 is sent from the first column to the top of the second column 102. A nitrogen-rich gas 41 is reheated in the subcooler 104 and then in the exchanger 9.

An oxygen-rich liquid 43 containing at least 97% of oxygen is withdrawn from the bottom of the second column 102, vaporized in the exchanger 45 at the pressure of the second column then compressed in a compressor 47.

A portion 17 of the nitrogen withdrawn from the top of the first column 100 is compressed in a compressor 19 without having been reheated, then is condensed in the bottom vaporizer-condenser 21.

The remainder 23 of the nitrogen is reheated in the exchanger 9 up to an intermediate temperature thereof, is expanded in a turbine 27, is reheated from the cold end of the exchanger to the hot end in order to form a nitrogen-rich gas.

Unlike FIG. 1, FIG. 2 comprises an argon column 106 which is used to separate the argon present in the second column and thus improve the purity of the oxygen or to enable more nitrogen to be withdrawn from the top of the medium-pressure column, which makes it possible to maximize the refrigeration available for the cold compressor. The column 106 is supplied by an argon-enriched flow 31 and produces a liquid flow 33 which is sent back to the second column. The argon column has an overhead condenser cooled conventionally by a portion of the liquid 35 (not illustrated). The argon-enriched gas 39 is mixed with the nitrogen-rich gas 41 in order to form a waste.

Two air flows are sent to the exchanger 9, the flow 1 being split into a flow 3 and a flow 7. The flow 3 is at the pressure of the first column and enters this column in gaseous form. The flow 7 is supercharged in a supercharger 5 to a higher pressure, cooled in the exchanger and then condensed in a vaporizer 11 where the liquid oxygen is vaporised to form gaseous oxygen 45.

FIG. 3 differs from FIG. 2 in that the gaseous oxygen is not compressed downstream of the exchanger 9. A portion 51 of the liquid oxygen is used as liquid product and the rest 43 is sent to the top of an auxiliary column 108 having a bottom reboiler 121 but without an overhead condenser. The air 7 is used to heat the bottom reboiler 121 of the auxiliary column 108. The gaseous oxygen 57 having to be used as product is withdrawn from the top of the auxiliary column 108 and is used as gaseous product 57. The bottom liquid is enriched in krypton-xenon and can be used to produce these gases.

It is possible to carry out the process from the three figures with two operating modes. According to the first operating made, the compressor 19 is shut down and the pressure of the air 1 is increased so that the reboiler 21 receives nitrogen at the pressure of the column 100 and at least one liquid is withdrawn from the system of columns as final product (for example liquid oxygen 51 and/or liquid nitrogen 38). During the second operating mode, the column 100 operates at a lower pressure than during the first operating mode. The compressor 19 operates and no liquid is withdrawn from the system of columns as final product or at least three times less liquid is withdrawn from the system of columns as during the first operating mode. Thus, the nitrogen used for heating the reboiler 21 is at the same pressure in both operating modes.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary a range is expressed, it is to be understood that another embodiment is from the one.

Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such particular value and/or to the other particular value, along with all combinations within said range.

All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited. 

1-13. (canceled)
 14. A process for separating air by cryogenic distillation in a system of columns comprising at least a first column operating at a first pressure and a second column operating at a second pressure below the first pressure, the top of the first column being thermally coupled to the bottom of the second column by a vaporizer-condenser wherein: i) cooling purified air in an exchange line and sending the purified air to the first column under conditions effective for separation; ii) withdrawing a nitrogen-enriched gas from the top of the first column and divided into two portions, a first portion is compressed in a compressor having an inlet temperature of at most −150° C. and is condensed in the vaporizer-condenser; iii) reheating a second portion of the nitrogen-enriched gas in the exchange line and expanded in an expansion turbine; iv) sending an oxygen-enriched liquid or a liquid derived from this liquid from the bottom of the first column to the second column in order to be separated therein, without having been reheated against a nitrogen-enriched gas from the first column; v) sending a nitrogen-enriched liquid from the top of the first column to the top of the second column; vi) withdrawing an oxygen-rich fluid from the bottom of the second column and is reheated in the exchange line; and vii) withdrawing a nitrogen-rich gas from the upper part of the second column and is reheated in the exchange line; wherein the vaporizer-condenser being the only means of vaporizing liquid in the lower part of the second column, wherein the second portion of the nitrogen-enriched gas is reheated in the exchange line and is expanded in the turbine, without having been compressed.
 15. The process as claimed in claim 14, wherein the oxygen-rich fluid contains at least 97% oxygen, or even at least 99% oxygen.
 16. The process as claimed in claim 14, wherein the nitrogen-enriched liquid sent from the first column to the second column is less rich in nitrogen than the nitrogen-enriched gas sent to the vaporizer-condenser.
 17. The process as claimed in claim 14, wherein all the gas condensed in the vaporizer-condenser is sent back as reflux to the first column.
 18. The process as claimed in claim 14, wherein the oxygen-rich fluid from step v) is an oxygen-rich gas.
 19. The process as claimed in claim 18, wherein the oxygen-rich gas is compressed after reheating in the exchange line to its production pressure.
 20. The process as claimed in claim 14, wherein the argon-enriched gas is withdrawn from the second column and sent to a third column in order to produce a flow richer in argon than the argon-enriched gas and a nitrogen-rich gas is withdrawn from the first column as product.
 21. The process as claimed in claim 14, wherein the compressor is driven by a motor and/or by a turbine that expands air or a nitrogen-enriched flow originating from the system of columns.
 22. The process as claimed in claim 14, according to first and second operating modes wherein, during the first operating mode, the compressor is shut down and at least one liquid is withdrawn from the system of columns as final product and during the second operating mode, the compressor operates and no liquid is withdrawn from the system of columns as final product or at least three times less liquid is withdrawn from the system of columns as during the first operating mode.
 23. An apparatus for separating air by cryogenic distillation that has a system of columns comprising at least a first column operating at a first pressure and a second column operating at a second pressure below the first pressure, the top of the first column being thermally coupled to the bottom of the second column by a vaporizer-condenser, an exchange line, means for sending purified air to be cooled into the exchange line and for sending the cooled air to the first column in order to be separated therein, means for withdrawing a nitrogen-enriched gas from the top of the first column, a compressor for compressing a first portion of the nitrogen-enriched gas having an inlet temperature of at most −150° C., means for sending a second portion of the nitrogen-enriched gas to the exchange line, a turbine for expanding the second portion of the nitrogen-enriched gas that is connected to the exchange line, means for sending the compressed gas to be condensed into the vaporizer-condenser, means for sending an oxygen-enriched liquid or a liquid derived from this liquid from the bottom of the first column to the second column in order to be separated therein, means for sending a nitrogen-enriched liquid from the top of the first column to the top of the second column, means for withdrawing an oxygen-rich fluid from the bottom of the second column, means for sending the oxygen-rich fluid to be reheated into the exchange line, means for withdrawing a nitrogen-rich gas from the upper part of the second column and for sending the gas withdrawn to be reheated into the exchange line, any means for reheating the oxygen-enriched liquid sent to the second column against a nitrogen-enriched gas from the first column and the vaporizer-condenser being the only means of vaporizing liquid in the lower part of the second column.
 24. The apparatus as claimed in claim 23, comprising an oxygen compressor for compressing the oxygen-rich gas, after reheating in the exchange line, to its production pressure.
 25. The apparatus as claimed in claim 23, comprising a third column , means for sending an argon-enriched gas from the second column to the third column, means for withdrawing a flow richer in argon than the argon-enriched gas from the third column and means for withdrawing a nitrogen-rich gas from the first column as product.
 26. The apparatus as claimed in claim 23, wherein the compressor is driven by a motor and/or by a turbine that expands air and/or the turbine that expands the second portion of the nitrogen-enriched flow. 